In recent years, absorbent resins which are capable of absorbing some tens to some hundreds times their own weights of water have been developed. Various absorbent resins of this class have been finding utility in applications requiring absorption and retention of water such as in the field of hygienic materials including sanitary goods and disposable diapers, the field of agriculture and horticulture, the field of foodstuffs requiring retention of freshness, and the field of industrial products requiring protection against dew formation and retention of refrigeration.
The absorbent resins of this class known to the art include the hydrolyzate of a starch-acrylonitrile graft polymer (JP-B-49-43,395), the neutralized starch-acrylic acid graft polymer (JP-A-51-125,468), the saponified vinyl acetate-acrylic ester copolymer (JP-A-52-14,689), the hydrolyzate of an acrylonitrile copolymer or acrylamide copolymer (JP-B-53-15,959), the cross-linked derivatives thereof, the self-crosslinking sodium polyacrylate obtained by reversed-phase suspension polymerization (JP-A-53-46,389), and the partially neutralized cross-linked polyacrylic acid (JP-A-55-84,304), for example.
The behaviors that the absorbent resins are required to possess vary depending on the kinds of use to be found for the absorbent resins. As the properties required of the absorbent resins which are intended for hygienic materials, high absorption capacity under load, high speed of absorption, and excels in fluid permeability to be manifested on contact with an aqueous liquid may be cited. These properties do not necessarily show positive correlations. It has been difficult to have these properties simultaneously adapted for one another.
In an effort to heighten the speed of absorption of an absorbent resin, attempts are being made to enlarge the surface area of the resin by such means as, for example, decreasing the particle size of the resin, granulating the resin, or molding the resin in the form of scales. Generally, when an absorbent resin is molded in the form of particles of a small size, the absorbent resin particles on contact with an aqueous liquid form the so-called "wet clusters" and incur a decrease in the speed of absorption. When an absorbent resin is granulated, the granules on contact with an aqueous liquid individually succumb to the phenomenon of being converted into "wet clusters" and rather suffer a decrease in the speed of absorption. When an absorbent resin is molded in the form of thin flakes, though the thin flakes are improved in terms of the absorption speed, the improvement is not fully satisfactory because the thin flakes induce the phenomenon of gel blocking. Further, the formation of thin flakes of the absorbent resin is uneconomical because the produced thin flakes inevitably assume a large volume and incur a large cost for transportation and storage.
Techniques for exalting the absorption speed of an absorbent resin by cross-linking molecular chains in the surface region of the absorbent resin and heightening the cross-link density in the surface layer and thereby precluding the formation of wet clusters have been proposed. These techniques are disclosed in JP-A-57-44,627, JP-A- 58-42,602, JP-B-60-18,690, JP-A-58-180,233, JP-A-59-62,665, and JP-A-61-16,903, for example. The improvement of absorbent resins in absorption speed has been attained to a certain extent by these techniques. The particles of these absorbent resins in their existent state contain particles of smaller size than the optimum size in a fairly high extent. When such a technique for improvement is used on an absorbent resin of such particle size distribution as described above, however, the treated absorbent resin acquires no amply high absorption speed and suffers a decrease in fluid permeability due to gel blocking.
For the solution of these problems, the following methods have been proposed for the granulation of absorbent resin. As techniques for granulation, (a) a method which comprises molding a fine absorbent resin powder with water or a binder combining water with a water-soluble polymer or inorganic powder (JP-A-61-97,333 and JP-A-61-101,536), (b) a method which comprises dispersing a fine absorbent resin powder in a non-aqueous liquid, causing the resultant dispersion to absorb an aqueous monomer solution added thereto, and polymerizing the monomer in the resultant mixture (JP-A-62-230,813), and (c) a method which comprises preparing an organic solvent containing the hydrogel of absorbent resin by the reversed-phase suspension polymerization technique, adding a fine absorbent resin powder to the hydrogel solution, and then subjecting the resultant mixture to azeotropic dehydration (JP-A-63-210,108) and as techniques for reclamation of a fine powder, (d) a method which comprises swelling a fine absorbent resin powder with water of an amount sufficient for forming amorphous gel, pulverizing the resultant continuous amorphous gel, and drying the pulverized gel (U.S. Pat. No. 4,950,692), (e) a method which comprises adding an aqueous cross-linking agent solution to a fine absorbent resin powder thereby forming gellike lumps, causing the resin in the lumps to undergo cross-linkage, and pulverizing the lumps of cross-linked resin (EP-A-0,401,004), and (f) a method which comprises allowing sprayed water to contact a dried fine powder thereby inducing partial hydration of the powder, kneading the fine powder with the water at the same time that the water is sprayed, and then in a separate area of mixture, kneading a viscous material with the hydrated fine powder at medium shear such that a substantially uniformly blended product results (EP-A-0,417,761), for example, have been known in the art.
In the case of the method of (a) mentioned above, the produced granules have no ample strength and consequently, during the course of conveyance and transportation along production lines in a plant, sustain partial fracture and possibly induce regeneration of a fine powder. Further, the granules, after absorbing a liquid, sustain fracture and regenerate a fine powder and consequently fail to acquire sufficient fluid permeability. In the case of the method (b) mentioned above, the absorbent resin is not easily made to absorb the aqueous monomer solution uniformly throughout the entire volume thereof and, as a result, the absorbent resin produces its granules only at a low ratio. Further, since the aqueous monomer solution permeates the resin powder before it undergoes polymerization, the absorbent resin consequently incurs a decrease in the absorption capacity. In the case of the method (c) mentioned above, although the granules are obtained indeed as contemplated, they are deficient in physical integrity so that, after being swelled with absorbed water, they induce regeneration of minute gel particles. Thus, this method is problematic in giving a product which merely impairs fluid permeability and fluid diffusibility.
The methods of (d), (e), and (f) mentioned above prove not only complicated and difficult in terms of operation but also disadvantageous in terms of economy because they require the fine absorbent resin powder to assume a swelled state by addition of water or an aqueous cross-linking solution and subsequently to resume a dry state.
The dry particles which are formed solely of the absorbent resin powder obtained by the method (d) mentioned above are as deficient in fluid permeability and fluid diffusibilityas the product of the method (c) mentioned above because these dry particles, when swelled with an absorbed liquid, cause substantially all the used fine powder to regenerate minute gel particles. In the case of the method of (e) mentioned above, since the cross-linking agent permeates together with a large amount of water to the cores of the minute particles of absorbent resin and undergoes a cross-linking reaction, the produced dry particles are at a disadvantage in incurring a decrease in the absorption capacity.
As means for producing an absorbent resin which forms a fine absorbent resin powder improved in various properties, a method which comprises mixing a carboxyl group-containing absorbent resin powder with a polyhydric alcohol and allowing them to react until the absorption capacity reaches a level in a fixed range (JP-A-04-214, 735), a method which comprises carrying out the procedure of this method in a high-speed stirring type mixer lined with a specific material (JP-A-04-214,734), a method which comprises causing the blend produced by the high-speed stirring type mixer to be heated under application of a fixed magnitude of force (energy) (JP-A-04-214,736), and particulate, absorbent, polymeric compositions (-JP-A-05-506,263) have been disclosed, for example.
Though these methods are indeed capable of improving absorption capacity under load, speed of absorption, and fluid permeability, they necessarily encounter difficulty in giving a fixed size to the produced particles because they use a high-speed stirring type mixer. They are, therefore, incapable of producing particles of an arbitrary size.
For the purpose of improving the fastness of adhesion of an absorbent resin to a substrate of such material as non-woven fabric or film or molding the absorbent resin in such a stably fixed shape as film or foam sheet, a method which comprises mixing an absorbent resin powder with a polyhydric alcohol such as glycerin or ethylene glycol (optionally in combination with a foaming agent) in the same manner as described above and applying the resultant mixture to a substrate thereby forming a sheetlike absorbent material (JP-A-57-73,051) has been also known to the art. The idea of using in this method a low-temperature extruding device or a low-temperature extruding device provided with a heating die for the purpose of the application mentioned above has been also disclosed. The low-temperature extruding device mentioned above, however, is used for the purpose of obtaining a foamed sheet without requiring use of a foaming agent by thoroughly kneading the absorbent resin powder with the poly-hydric alcohol and uniformly dispersing minute air bubbles arising from a gas separately introduced into the device and the low-temperature extruding device provided with the heating die is likewise used for the purpose of obtaining a foamed sheet by thoroughly kneading the absorbent resin powder with the liquid organic polyhydroxy compound and the foaming agent and causing the resultant mixture to foam in the part of heating die. They are not aimed at obtaining granules of an arbitrary size.
As a means for obtaining particles of an arbitrary size, a method which comprises combining an absorbent resin with a mineral powder, an absorption inhibitor selected from the group consisting of alcohols, ketones, esters, ethers, aldehydes, and inorganic neutral salts, and water and extrusion molding the resultant mixture (JP-A-01-266,139) has been disclosed. Though this method permits fairly good control of particle size in the range of relatively rough particle size, it experiences difficulty in obtaining particle size, 0.1 to 1.0 mm, which are generally appropriate for hygienic materials. Further, this method incurs adulteration of the produced absorbent resin with a large proportion of defiling substances and, therefore, is incapable of obtaining an absorbent resin which excels in various absorption properties including absorption capacity and speed of absorption.
An object of this invention, therefore, is to provide a novel absorbent resin composition and a method for the production thereof.
Another object of this invention is to provide an absorbent resin composition featuring a high speed of absorption, excellent fluid permeability, and a high absorption capacity under load and a method for economically advantageous production thereof.
Still another object of this invention is to provide granules of absorbent resin permitting the arbitrary control of particle size that has never been attained by any of the conventional method, possessing high physical integrity, and excelling in ease of handling and various absorption properties including fluid permeability and speed of absorption and a method for economically advantageous production thereof.
A further object of this invention is to provide granules of absorbent resin which are relatively uniform in shape and size, capable of retaining shape while absorbing a liquid, and free from regeneration of fine particles.